1. Field of the Invention
The present invention relates generally to a process for fabricating ceramic fibers from water-soluble polymers, and more particularly to a process for fabricating strong, creep resistant Y.sub.3 Al.sub.5 O.sub.12 fibers.
2. Description of the Related Art
Since the development of E-glass fibers in the 1920's and their successful exploitation as reinforcements for metals and polymers in 1940's, a number of ceramic fibers have entered the composites market. The high modulus, specific strength, and thermo-oxidative resistance of these fibers have allowed the replacement of traditional monolithic components with composite parts of superior strength, reliability and fuel efficiency.
Ceramic fibers are manufactured by either the polymer or the colloidal methods. In the former, a thermoplastic inorganic polymer solution of suitable viscosity is spun and pyrolytically decomposed. Nicalon, a registered trademark of Nippon Carbon, and HPZ, a registered trademark of Dow Corning, are two commercially available fibers produced by this method.
In the colloidal method, a sol consisting of ultrafine ceramic particles and fugitive organic additives is spun and sintered. A number of commercially available fibers are produced with this method including PRD 166, a registered trademark of DuPont, and Almax, a registered trademark of Mitsui Mining Co.
Despite the success of ceramic fibers, none of the commercially available fibers can bear load at temperatures exceeding 1100.degree. C. in an oxidizing atmosphere, as evidenced by the data shown in FIG. 1. All the fibers were tested in air. It is clear that a new class of fibers must be developed for high temperature applications such as in advanced high temperature gas turbine engines, in which service temperatures in excess of 1300.degree. C. are foreseen.
The National Materials Advisory Board's Commission on Engineering and Technical Systems of the National Research Council recently stated that a review of the properties of the oxide ceramic fibers that are available at the present time discloses that they cannot meet many of the projected requirements for reinforcement of high-temperature composites requiring stable strength and stiffness properties and resistance to creep at high temperatures (e.g.&gt;1200.degree. C.), for extended periods of time. New fibers of selected compositions will be necessary to satisfy these needs.
Recent examinations of high temperature materials point to Y.sub.3 Al.sub.5 O.sub.12 (yttrium aluminate garnet) hereinafter referred to as YAG fibers as the best qualified material. Currently, both the polymer and colloidal methods previously discussed are being actively pursued to synthesize YAG fibers. For example, at least one company is pursuing a polymer route, while another a colloidal route.
FIG. 2 shows the excellent creep resistant properties of YAG compared to commercial fibers of conventional ceramic materials. Alumina fibers exhibit good creep resistance, but are not as corrosion resistant as evidenced by the data in Table 1. Table 1 compares these two types of fibers in both lithium (Li) and sodium (Na) environments.
TABLE 1 ______________________________________ Al.sub.2 O.sub.3 Y.sub.3 Al.sub.5 O.sub.12 ______________________________________ Li 375.degree. C. 12 mm/yr 0.016 mm/yr Na 1000.degree. C. Severe 0.788 mm/yr ______________________________________
Consequently, there is a need to develop an economical and commercially feasible process for fabricating YAG fibers including a fiber spinning technique therefor.